1. Field of the Invention
The present invention relates to a method and apparatus for recording data on a medium in general and to a method and apparatus comprising a laser having an extended cavity which includes the medium for recording data on the medium in particular.
2. Description of the Prior Art
Lasers have been used for recording data on magnetic as well as on nonmagnetic or optic recording mediums.
Currently, apparatus comprising lasers for recording data on a magnetic recording medium is known as a magnetooptic recording apparatus. To record data on a magnetic medium, such apparatus uses what is called the thermomagnetic writing effect.
Referring to FIG. 1 of the accompanying drawing, there is provided a block diagram of a typical prior known magnetooptic recording apparatus. In this apparatus, laser light is generated by a laser 101, collimated by a lens 102 and focused by a lens 103 on a magnetic recording surface 106 of a disk 105 which is rotatable about an axis 108. An apparatus represented by crossed arrows 104 is provided for moving the lens 103 to keep the light focused at the surface 106, as well as to allow the focus point to be moved small increments in the radial direction of the disk. A means of inducing a magnetic field in the region of the focus point is provided by a coil 109. The disk 105 may also be provided with a transparent overcoat 107 to protect the surface 106.
In operation, the laser light focused on the surface 106 lowers the coercivity of a region of the underlying magnetic material. A polarized magnetic field generated by the coil 109 is then used for inducing a magnetic domain in the region. Logical ones and zeros are thus stored depending on the polarity of the magnetic field.
Because the magnetic medium is not an integral part of the laser optical cavity, a principal disadvantage of the prior known laser magnetooptic recording apparatus is the requirement for a lens system and focus control. The lens system and focus control are disadvantageous because they are typically expensive and require a great deal of space in the apparatus. The lens system, in particular, is disadvantageous due to problems associated with diffraction and other optical phenomena and the consequent restriction on the amount of energy that can be concentrated within a given area of the surface of the recording medium as a result thereof.
An apparatus in which an external reflective member is an integral part or component of the laser optical cavity has been disclosed by R. O. Miles, A. Dandridge, A. B. Tveten and T. G. Giallorenti in their article "An External Cavity Diode Laser Sensor", which appeared in the Journal of Lightwave Technology, Vol. LT-1, No. 1, March 1983. In this apparatus, forming a semiconductor laser sensor, a front facet of a diode laser is arranged to be held within a few wave lengths of a reflective member. As the reflective member is perturbed, even slightly, the phase of the light reflected back into the laser cavity is altered thus varying the effective laser facet reflectivity. The change in laser output is detected as a measure of the distance the reflective member moves relative to the exit facet of the laser.
While the article describes an external or extended cavity laser in which a reflective member is an integral part of the laser cavity, the description is restricted to an apparatus responsive to phase changes in reflected light in the optical cavity. There is no disclosure or suggestion of any use of such a laser for recording on a recording medium of any kind.
Prior known apparatus used for optic disk writing is similar to the apparatus described above for magnetooptic writing, except that the recording medium is typically nonmagnetic and consequently the coil 109 is not needed.
In the operation of optic disk writing apparatus comprising a laser, the focused output of the laser is used to heat a spot on the disk surface until some change of state on the surface of or in a layer on the disk occurs. This change may comprise the melting, pitting or blistering of the layer or the facilitating of a chemical reaction or phase change in the layer. In any case, the resolution of prior known optic disk writing apparatus which uses a lens system, as in the case of prior known magnetooptic recording apparatus, is limited by the capability of the lens system to focus the light down to a very small spot, and to keep the focal plane at the surface.